The spawner wont spawn a mob that was made with Bos but works perfectly for a normal Entity when put into the same spawner.

Just flew to Map Spawn and they were all spawning at Spawn. So they are spawning but spawning in the wrong area

This happens when you use /bos getempty to pick up a mob and put it in a BoS, then you put that mob into a spawner with /nbts additem.

The problem is that the empty book picks up all mob NBT data, including position. So when the mob spawns, it will spawn with the same position as the picked-up mob, even though your spawner is placed elsewhere.

To fix this, you need to run /bos clearvar Position before adding it to a spawner. You may also want to clear Velocity and Rotation, depending on what you're doing.

This is not an NBTEditor bug. @goncalomb should close this.